Critical distance is where direct sound energy equals reverberant sound energy in a room

In a reverberant room, direct sound (which follows the inverse-square law) decreases with distance while reverberant sound stays roughly constant throughout the room. Critical distance is the specific distance from the source where direct and reverberant levels are equal. Beyond critical distance, the reverberant field dominates and increasing the speaker level only raises the room noise, not intelligibility. Directional loudspeakers extend the critical distance by concentrating energy in the listening direction rather than exciting the room evenly. For speech intelligibility, the audience should ideally be within the critical distance of the loudspeaker system.

Examples

A directional loudspeaker system has a critical distance of 30 feet; audience rows behind 30 feet are dominated by reverberant energy and will experience reduced intelligibility. Using distributed overhead speakers can bring each listener within the critical distance of a closer source.

Assessment

Why does using a more directional loudspeaker increase critical distance? Why can’t you solve a reverberation problem just by turning the system up?